aJsonClientStream::getch(): Fix to work with WiFiClient too

EthernetClient will stay connected() as long as data is available(),
but WiFiClient() can switch to !connected() while there are still data
unread in its buffer and therefore available(). Therefore, let's check
for available() when reading data rather than connected().
Merge pull request #44 from pasky/master

Fix aJsonClientStream to work with WiFiClient too